1. Field of the Invention
The invention relates to a method for doping a conjugated polymer. Polymers preparable according to the method of the invention are provided.
2. Description of Related Technology
Doping of conjugated polymers (polymers with pi-conjugated backbone structures and/or pi-conjugated pendant groups) with strong protonic acid (p-doping) or strong oxidizing (p-doping) or reducing agents (n-doping) is well established in the literature. However, the doping proceeds readily to completion in the presence of a stoichiometric or excess amount of dopants. The chemical driving force for maximum doping is very high, so that it is difficult to arrest the doping level at an intermediate value. The system achieves the maximum doping with about 10–50% of the conjugated repeat units doped depending on the polymer system. For poly(p-phenylenevinylenes) and polyacetylenes, this is typically 10–20%; for polythiophenes, 20–30%; for polyanilines, 40–50%. This maximum level of doping imparts a high level of electrical conductivity of the order of 1–1000 S/cm to the polymers, depending on the nature and type of the polymers and dopants used, so that they become conducting polymers in the process. The bulk carrier concentration is then roughly of the order of 1020 /cm3 to 1021 /cm3.
However, this high level of doping is unnecessary or even undesirable for some applications. For example, for a 1-μm thick film (which is typical of the vertical thickness of photonic structures) having a conductivity of 10−6 S/cm, only a modest 1-V potential difference is required to drive a practical device current density of 10 mA/cm2 through the film thickness direction. Therefore, film conductivities of the order of 10−6–102 S/cm (typical of the semiconducting range) are already sufficient for these films to be employed in semiconducting photonic structures such as distributed Bragg reflectors and waveguides.
Furthermore, when the films are doped to the maximum, such as achieved by straightforward exposure to strong acids or oxidants, their optical properties change in drastic ways owing to the formation of new sub-gap transitions that change the refractive indices of the films and cause parasitic absorption of any emitted light. Both these factors are not desirable or acceptable for photonic applications. Therefore control of the bulk carrier concentration between 1017 /cm3 to 1020 cm3, at an intermediate doping-level at least about one order of magnitude less than the maximally-doped case, is crucial.
Applied Physics Letters, volume 73, Number 2, pages 253–255 (1998) reports a study of the Hall mobility and the carrier concentration of a conjugated polymer, namely polythiophene, as a function of the electrochemical doping level. The doping level of the polymer is changed by varying the oxidation potential i.e. by potentiometric control.
Synthetic Metals, 68, pages 65–70 (1994) is concerned with field-effect mobility and conductivity data obtained from two different amorphous organic semiconductors which can be doped to a range of different conductivities.
Synthetic Metals, 89, pages 11–15 (1997) investigates the doping and temperature dependence of the conductivity of poly(p)-phenylene vinylene (PPV).
Synthetic Metals, 55–57, pages 3597–3602 (1993) investigates electrical conductivity of α,α-coupled dodecathiophene as a function of both dopant level and time.
Synthetic Metals, 30, pages 123–131 (1989) discloses a relationship between acid strength and ionization potential of a conjugated polymer that will give a highly conductive doped complex.
Applied Physics Letters, volume 72, pages 2147–2149 (1998) describes a doped hole transporting polymer. Differing levels of doping are realized by adjusting the co-evaporation rates of polymer and dopant material.
The methods used to achieve different levels of doping in the above systems are not satisfactory for controlling the doping level to such a degree so that a balance between optical and electrical property of the doped polymer can be struck.